1. Field of the Invention
The present invention relates to a negative electrode for a lithium secondary battery and also relates to a lithium secondary battery comprising the negative electrode.
2. Discussion of Background
Recent development of small, thin, and lightweight electronic appliances in remarkable, particularly in the field of office automation. For instance, lightweight electric appliances have been developed from desk-top type to lap-top typo, and then to note-book type.
In addition, new small size electronic appliances such as electronic notebooks, and electronic still cameras are also placed on the market. Furthermore, memory cards which are now small size memory media are now being developed in addition to the reduction in size of conventional hard disks and floppy disks.
In the midst of the recent trend of making small sized, thin and lightweight the above-mentioned electronic appliances, a secondary battery with higher performance capable of supporting these appliances is demanded.
Under such circumstances, a lithium secondary battery has been rapidly developed as a battery with high energy density, by which conventional lead storage batteries and nickel cadmium storage batteries will be eventually replaced. (A) As an active material for use in a positive electrode (hereinafter referred to as the positive electrode active material) for such a lithium secondary battery, transition metal oxides and transition metal chalcogenides such an TiS.sub.2, MoS.sub.2, CoO.sub.2, V.sub.2 O.sub.5, FeS.sub.2, NbS.sub.2, ZrS.sub.2, VSe.sub.2, and MnO.sub.2 are used. Many secondary batteries using such inorganic materials as active materials have been studied.
When such inorganic materials are used as active materials, it is possible to perform reversible, electrochemical intercalation of lithium ions into and deintercalation thereof from the structure of these inorganic materials. By utilizing this property of the above inorganic active materials, the development of lithium secondary batteries has bean made.
Generally, lithium secondary batteries using the above-mentioned inorganic materials as positive electrode active materials can easily constitute secondary batteries having high energy density because those positive active materials themselves have high densities.
Furthermore, in the case where occlusion and releasing of lithium ions are respectively carried out by the intercalation of lithium ions into the crystalline structure of the above positive electrode active material, and by the deintercalation of lithium ions from the crystalline structure of the positive electrode active material, a battery with excellent voltage plateau can be easily fabricated. Such a battery, however, has the shortcoming that when lithium ions are excessively built up in the crystalline structure of the positive electrode active material, the crystalline structure of the positive electrode active material is destroyed and the function thereof is significantly degraded. This means that such inorganic materials are vulnerable to overdischarge when used as positive active materials for the lithium secondary battery.
Recently during the development of lithium secondary batteries using such inorganic materials as positive electrode active materials, conducting polymers have been discovered which can perform an electrode reaction by carrying out reversible occlusion and releasing or doping and undoping of an anion and therefore can be used as positive electrode active materials for a lithium secondary battery.
Such conducting polymers have the advantages over conventionally employed inorganic materials that they are light and exhibit high output power density, excellent electric collection performance due to the electroconductivity thereof, high cycle characteristics for a 100% depth of discharge, and excellent workability for the fabrication of an electrode.
Examples of such conducting polymers so far reported are polyacetylene (refer to, for example, Japanese Laid-Open Patent Application 56-136499), polypyrrole (refer to, for example, the 25th Battery Symposium, Abstracts, P2561.1984), and polyaniline (refer to, for example, the 50th Convention of Electric Science Association, Abstracts, P2281.1984).
For instance, in Japanese Laid-Open Patent Application 63-102162, there is proposed a composite electrode comprising a conducting polymer and an inorganic active material.
Japanese Patent Application 5-129997 discloses a positive composite electrode with high energy density which is fabricated by composing a conducting polymer and an inorganic material under particular conditions, which has excellent workability, voltage plateau, and current characteristics.
(B) Another problem encountered in the course of the development of the lithium secondary battery is the development of a negative electrode. Conventionally, as a negative active material for the negative electrode of a lithium secondary battery, lithium and lithium-aluminum alloys are used.
When lithium is employed as the negative active material for the negative electrode, high electromotive force can be provided, and high energy density can also be provided because of the light weight of lithium. However, lithium has the shortcoming of shortening the cycle life of the battery because of the formation of dendrites thereof which decompose an electrolytic liquid employed in the battery. Further, when the dendrites grow and reach the punitive electrode, internal short-circuits take place in the battery.
When a lithium alloy is employed, the difficulties caused by the above-mentioned problems can be lightened. However, a secondary battery with satisfactory capacity cannot be obtained when a lithium alloy is employed as the negative active material for the negative electrode thereof.
Under such circumstances, it has been proposed to use as negative active materials carbon materials capable of occluding and releasing lithium ions. However, presently announced batteries which employ carbon materials in the negative electrodes thereof are not considered to make best use of the function of a lithium ion battery. It is considered that this is because the energy density and the chargeable and dischargeable current densities at the negative electrode are not high. To be more specific, a carbon material with high crystallinity is theoretically expected to have an energy density of 372 mAh/g. In comparison with such a high energy density as inherently possessed by the carbon material used as a negative active material, the energy density of a worked negative electrode, in particular, the volume energy density thereof, is significantly lower, and the internal impedance of the electrode is so high that the energy density is high at a low current density, but the energy density at the negative electrode is considerably decreased when charging and discharging are performed with a large electric current.
Furthermore, changes in the crystalline structure of the carbon materials, caused in the course of the charging and discharging steps, are so large that the strength of the negative electrode is lowered during the repetition of charging and discharging cycles, resulting in insufficient cycle characteristics.
Furthermore, even if the initial capacity of the negative electrode is large, the negative electrode deteriorates during the repeated charging and discharging cycles, and the capacity thereof rapidly decreases. Due to these problems, the conventional carbon materials are not suitable for making satisfactory use of the function of a secondary battery.
As the above-mentioned carbon materials, carbons with high crystallinity are preferable. In particular, graphite is preferable. However, when graphite is used alone, at present, satisfactory results are not always obtained due to the problems in connection with the matching with an electrolytic liquid, the capacity of the negative electrode, and cycle characteristics of the battery.